1. Field of the Invention
The invention is concerned with a polymerizable composition which can be polymerized and cured by light or heat, a planographic printing plate precursor using the same, and a planographic printing method using the same. Specifically, the invention relates to a stable polymerizable composition capable of polymerizing with high sensitivity which is useful for use in a recording layer of a planographic printing plate precursor which can be used for printing without being alkali-developed after exposure to laser light. The invention also relates to a planographic printing plate precursor and a planographic printing method using the printing plate precursor, the method comprising on-press development but not comprising usual development operation.
2. Description of the Related Art
Generally, a planographic printing plate comprises lipophilic image portions receiving ink at printing and hydrophilic non-image portions receiving moistening water. The planographic printing comprises: forming ink-receiving portions which are lipophilic image portions and moistening water receiving portions (ink-repelling portions) which are hydrophilic non-image portions on a planographic printing plate so as to make irregularity of ink adhesiveness on the surface of the planographic printing plate; allowing ink to adhere only to the image portions utilizing the repulsion between water and oil-based ink; and transferring the ink to a member (such as paper) on which the image is to be printed to conduct printing.
Planographic printing plate precursors (PS plates) have been widely used for making planographic printing plates, which printing plate precursors have a hydrophilic support and a lipophilic photosensitive resin layer (image-recording layer) on the support. Usually, the planographic printing plate precursor is exposed to light which has passed an element having an image such as a lith film, and then the image-recording layer in non-image portions is removed by being dissolved in an alkaline developer or an organic solvent while allowing the image-recording layer in image portions to remain so that the surface of the hydrophilic support is partially exposed in accordance with the image to form a planographic printing plate.
Conventionally, recording materials each having a resin layer which can be polymerized and cured by light or heat have been widely used as recording materials for planographic printing plate precursors.
Digitalization techniques have been spreading widely which process, store, and output image information electronically by using computers. In accordance with the development of digitalization techniques, new image out-put methods have been put into practice which are adapted to such digitalization techniques. As a result, there is a demand for computer-to-plate (CTP) techniques which comprise scanning a printing plate precursor with a light with high directivity such as a laser light according to digitalized image information, thereby directly producing a printing plate without using a lith film. Accordingly, it has been an important issue to develop an image-recording material adapted for such techniques.
An image-recording material proposed as a scan-exposable recording material has a constitution in which a hydrophilic support has thereon a lipophilic photosensitive resin layer (hereinafter occasionally referred to as “photosensitive layer”) including a photosensitive compound capable of generating active species such as radicals and Bronsted acids upon exposure to laser light. This image-recording material is commercially available as a planographic printing plate precursor. A negative planographic printing plate can be obtained by: scanning the planographic printing plate precursor with a laser light according to digital information to generate active species which causes chemical or physical changes of the photosensitive layer, thereby insolubilizing the photosensitive layer; and then developing the planographic printing plate precursor.
Specifically, a planographic printing plate with high resolution and high ink-affinity can be obtained efficiently by a simple developing treatment when the planographic printing plate precursor has a hydrophilic support and a photopolymerizable photosensitive layer (recording layer) containing a photopolymerization initiator excellent in response speed, an addition-polymerizable ethylenic unsaturated compound, and a binder polymer which is soluble in an alkaline developer, and an optional oxygen-blocking protective layer on the support. A planographic printing plate having the desired characteristics can be produced from such a planographic printing plate precursor.
A highly-sensitive radical-polymerizable recording material suitable for use as a recording layer of a planographic printing plate usually comprises a combination of an alkali-soluble polymer binder and a polymerizable crosslinking agent (monomer or oligomer). In order to improve sensitivity of a polymerizable composition containing the combination, techniques have been proposed; a technique disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-187322 provides a radical polymerizable group to the binder, and a technique disclosed in JP-A No. 2001-290271 introduces a phenyl group substituted by a vinyl group to a side chain of the binder. When the binder polymer has a radical polymerizable group, the binder, which is a high-molecular-weight compound, can directly participate in cross-linking, thus enhancing the cross-linking effects by molecular-weight effects and improving the sensitivity. However, the introduction of the polymerizable group makes thermally instable the polymer and the polymerizable composition containing the polymer. The image quality is deteriorated when such techniques are applied; particularly, the polymerizable composition may deteriorate because of the high gelling efficiency of the cross-linking binder when stored in a condition of high temperature and high humidity, and decrease in developability of a non-image portion may cause poor reproducibility of thin lines if the non-image portion is a space between the thin lines. Therefore, there has been a need for a composition which cures with high sensitivity to form a tough film only in exposed portions while maintaining high removability of non-exposed portions.
In conventional techniques, a process for making a planographic printing plate has to have a process for removing non-image portions after exposure; the non-image portions are removed by being dissolved in a developer suitable for the image-recording layer. It has been an issue to eliminate the need for the additional wet process described above or simplify the wet process. Particularly, the industry has paid attention to the disposal of the waste liquid occurring in the wet process, in consideration of the global environment; accordingly, there is a stronger request for solving the above problems.
As a simple method for making a printing plate which can solve the problems, an on-press development method has been proposed in which the recording layer is such a recording layer that non-image portions of the planographic printing plate precursor can be removed in a usual printing procedure, and in which the non-image portions is removed on the printing machine to form a planographic printing plate.
Specifically, the on-press development may be carried out in the following manners: a method using a planographic printing plate precursor having such an image-recording layer as to be dissolved or dispersed in an emulsion containing the moistening water and the solvent of the ink or in an emulsion containing the moistening water and the ink; a method comprising physically removing the image-recording layer by the contact between the image-recording layer and the rollers or blanket cylinder; and a method comprising weakening the cohesive force of the image-recording layer or the adhesion force working between the image-recording layer and the support by penetration of the moistening water and the ink solvent into the image-recording layer, and physically removing the image-recording layer by the contact between the image-recording layer and the rollers or blanket cylinder.
In the invention, the term “developing treatment” refers to a treatment in which: a planographic printing plate precursor which was exposed to infrared laser light or the like is brought into contact with a liquid (usually an alkaline developer) using an apparatus (usually, an automatic processor) which is not a printing machine such that non-exposed portions are removed to expose the surface of the hydrophilic support. In the invention, the term “on-press development” refers to a method or treatment in which a planographic printing plate precursor is brought into contact with a liquid (usually a non-alkaline aqueous solution such as printing ink and moistening water) using a printing machine such that non-exposed portions of the planographic printing plate precursor is removed to expose the surface of the hydrophilic support.
A planographic printing plate precursor disclosed in Japanese Patent 2938397 etc. is an image-recording material capable of on-press development which has, on a hydrophilic support, an image-recording layer containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder. The planographic printing plate precursor can be developed on-press with moistening water and/or ink and exhibits excellent on-press developability; the planographic printing plate precursor is exposed to a laser light to fuse the thermoplastic polymer particles to form an image and then attached to the cylinder of a printing machine, and then developed on-press. However, when the image is formed by mere thermal fusion of the particles, the strength of the obtained image is poor and printing durability is insufficient; particularly, the adhesion between the support and the ink-receiving layer is very weak.
A planographic printing plate precursor having, on a hydrophilic support, microcapsules containing a polymerizable compound has been proposed for example in JP-A Nos. 2001-277740 and 2001-277742.
Further, a planographic printing plate precursor having, on a support, a photosensitive layer containing an infrared absorber, a radical polymerization initiator, and a polymerizable compound has been proposed for example in JP-A No. 2002-287334.
Another planographic printing plate precursor capable of on-press development has been disclosed for example in JP—No. 2002-287334. This planographic printing plate precursor has, on a support, a photosensitive layer containing an infrared absorber, a radical polymerization initiator such as an onium salt or an organic borate compound, and a polymerizable compound having an ethylenic unsaturated double bond such as acrylic ester.
The image portion formed by the polymerization has a higher denseness of chemical bonds in the image portion than the image portion formed by the thermal fusion of polymer particles. Therefore, the image portion formed by the polymerization has a better image strength; however, even such a planographic printing plate precursor fails to achieve sufficient on-press developability, printing durability, and polymerization efficiency (sensitivity) from the practical viewpoint, whereby the planographic printing plate precursor has not been put into practice.
Therefore, there is a need for a polymerizable composition which has high sensitivity and curability and which enables easy removal of non-cured portions with a non-alkaline water or the like without requiring removing treatment with an excessive amount of developer.